I experienced the same lock screen flickering issue with my L920.
Bought 3 days back.
With 1st device started facing the issue within 45 minutes of opening the box. I doubt as I started doing the wireless charging and when it goes hot it started giving this problem.
Returned the device. Got the 2nd handset using since last night started the same problem again this afternoon.
I got one personal call, went outside to take the call when I droped the call checked status update. Then lock the screen and bang on started the ******* problem again.
To regenerate the issue, I did the same went inside kept the mobile idle for few mins until it gets cool, then again went in direct sunlight, tried browsing and after 5 minutes locked the screen then the moment I get off the sunlight the problem started.
So the reason it is pretty obvious it has some heating issue, when ever it gets hot the screen gets started flickering.
This is really shame for Nokia, I don't expect this from Nokia. Before this I had L925 in that I didn't faced such issues, it is only with this L920.
Any how left with no other option rather than to wait for new release of l device.